Method and arrangement for forming an address

ABSTRACT

A method and arrangement for forming an address for use in signal predistortion. The arrangement is used to form a corrector table address employed in the signal predistortion to compensate for signal distortions. The signal distortions are compensated for using corrector coefficients that are placed in the corrector table and retrieved from the table on the basis of the address. The arrangement includes calculation means that, on the basis of the received signal, calculate a result corresponding to the squaring of a received signal. The arrangement further comprises summing means that, if required, sum up the results corresponding to the squaring, the sum forming a base address. The arrangement further comprises error correction means that correct the calculated base address by means of an address correction value.

FIELD OF THE INVENTION

The invention relates to a method for forming an address, which methodis employed in signal predistortion used for compensating for signaldistortions by means of corrector coefficients placed in a correctortable, the corrector coefficients being retrieved from the correctortable on the basis of the address.

The invention also relates to an arrangement for forming a correctortable address used for signal predistortion which is used forcompensating for signal distortions by means of corrector coefficientsplaced in a corrector table, the corrector coefficients being retrievedfrom the corrector table on the basis of the address.

DESCRIPTION OF THE PRIOR ART

Many electronic components bring about signal distortion when the signalis processed by the component. In practice, signal distortion isproblematic especially in amplifiers. Distortion in the amplifier iscaused by the non-linearity of the amplifier. The signal to be amplifiedcan also become distorted because of too strong an input signal orbecause of for example a shift in the standby operating point of theamplifier. It is possible to reduce distortion for example by afeedback.

Linear modulation methods that enable a spectrum to be used efficientlyare becoming more common in mobile systems. However, the linearmodulation methods require that the amplifier must be as linear aspossible. However, the power amplifiers in use cause intermodulationdistortion to the signal. Intermodulation distortion occurs when theamplifier generates harmonic waves, i.e. new frequencies. Moreprecisely, intermodular distortion occurs when an excitation signalcomprises oscillation at different frequencies. In that case, theresponse comprises a number of signal whose frequencies are sums anddifferences of the excitation frequencies and their multifolds.

Distortion in the amplifier can further depend for example on themagnitude of capacitance and inductance in the amplifier. Mobile systemscontain for example AM-AM or AM-PM intermodular distortion (AM=AmplitudeModulation, PM=Phase Modulation). Distortion caused by intermodulationdistortion is presented for example by means of a signal state diagram.The signal state diagram can be presented by means of coordinates. TheAM-AM distortion causes signal state points to shift closer to theorigin. The AM-PM distortion, on the other hand, causes the signal statepoints to move round the origin.

Various linearization methods are used to correct and compensate for theamplifier-induced signal distortion. Prior art methods employ digitalpredistortion in linearization. In digital predistortion, a basebandsignal is corrected by means of correction coefficients stored in atable. The correction coefficients are retrieved on the basis of anaddress that has been calculated from an input signal. An input signalamplitude, i.e. the square root of the input signal, is used as theaddress. However, calculating the square root reduces calculationcapacity. In addition, implementing the calculation is complicated.

For example U.S. Pat. No. 5,138,144 presents an error compensationmethod associated with image reading equipment, the method comprisingaddress information calculations. However, calculating addressinformation is time-based. More precisely, calculating an address isbased on a clock pulse.

BRIEF DESCRIPTION OF THE INVENTION

The object of the present invention is to change the addressdistribution of a corrector table on the basis of the amplitudedistribution of an input signal.

This object is achieved by the method of the type presented in theintroduction, characterized by calculating a base address on the basisof a signal, summing up the signal components of a signal composed ofsignal components, correcting the calculated base address by an addresscorrection value, calculating a corrector table address on the basis ofthe base address and the corrected base address.

This object is also achieved by the arrangement of the invention,characterized in that the arrangement comprises calculation means that,on the basis of a received signal, calculate a result corresponding tothe squaring of the received signal, summing means that, if required,sum up the results corresponding to the squaring, the sum forming a baseaddress, and error correction means that correct the above calculatedbase address by means of an address correction value.

The method of the invention provides many advantages. The method enablesa correction table address to be formed without complicatedcalculations. According to the method, a base address is formed beforeforming the actual corrector table address. The actual address isobtained by suitably processing the base address, the actual addressthen being used as the corrector table address. Processing the baseaddress is carried out by using address correction means that correctthe above calculated base address. Processing is carried out by means ofaddress correcting values. The address correcting values arranged to theinput signal enable an optimal corrector table address to be formed. Theoptimal address enables a distortion to be accurately compensated for.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail with reference to theexamples in the attached drawings, in which

FIG. 1 is a schematic block diagram of a transceiver employing digitalpredistortion, and

FIG. 2 illustrates an arrangement used for forming a corrector tableaddress used for digital predistortion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram of a transceiver arrangement using digitalpredistortion. The arrangement comprises predistortion means 100,calculation means 110 and calculation means 120. In the arrangement ofthe figure, two signal components I(t) and Q(t) are introduced to thepredistortion means 100. The signal components are also applied to boththe calculation means 110 and 120. The calculation means 120 calculatecorrector coefficients.

The arrangement further comprises means 130 for establishing a correctortable. The corrector table comprises the corrector coefficientscalculated by the calculation means 120. The calculation means 110calculate an address on the basis of the received signal components, theaddress being used for retrieving the corrector coefficients from thecorrector table. The corrector coefficients are applied to thepredistortion means 100 that compensate for signal distortions by meansof the corrector coefficients.

The arrangement further comprises a converter element 131, a converterelement 132, filter means 141,142, and an oscillator 150. The converterelement 131 is connected to the predistortion means 100 that supplydigital signals to the converter element 131. The converter element 132is connected to the calculation means 120. The converter element 132receives analogue signals. The converter element 132 converts thereceived analogue signal into a digital signal that is sent to thecalculation means 120. The arrangement further comprises a modulator anda demodulator 161,162. The oscillator 150 generates a signal that isapplied to the modulator 161 and to the demodulator 162. The arrangementfurther comprises an amplifier 171 connected to the modulator 161. Thearrangement further comprises an antenna 181 acting as a transceiverantenna in the solution according to the figure.

The filter means 141 receive analogue signals arriving from theconverter element 131. The filter means 141 low-pass filter the receivedsignal. The signal filtered by the filter means 141 is applied to themodulator 161. In the modulator 161, a signal generated by an oscillator151 is modulated by the signal coming from the filter means 141. Themodulated signal is applied to the amplifier 171 that amplifies thesignal. After the amplification, the signal is sent through the antenna181 to the radio path.

The antenna 181 receives analogue signals from the radio path, thesignals upon receiving being applied to the demodulator 162. In thedemodulator, the signal generated by the oscillator 151 is modulated bythe signal coming from the antenna 181. The demodulated signal isapplied to the filter means 142 that filter the signal. After filtering,the signal is converted into a digital signal in the converter element132.

FIG. 2 shows an arrangement used for forming an address for thecorrector coefficients placed in a corrector table. The figure thuspresents the structure of the calculation means 110 in more detail. Thearrangement comprises calculation means 210, 220, and a summing means240. The summing means 240 is connected to the calculation means 210,220. The summing means receives signals from the means 210, 220. In thearrangement of the figure, the calculation means 210 receives a signalI(t). The calculation means 210 processes the received signal in such away that the strength of the signal obtained from the output of thecalculation means 210 is I²(t). The calculation means 220 receives asignal Q(t). The means 220 processes by preferably squaring the receivedsignal in such a way that the strength of the signal obtained from theoutput of the calculation means 220 is Q²(t). In practice, thecalculation means 210, 220 are implemented by a DSP technique.

The output signals I²(t) and Q²(t) of the calculation means 210, 220 areapplied to the summing means 240 that sums up the received signals. Thecalculation means 210, 220 generate an output signal for example bymultiplying their input signals by each other. The output signal canalso be generated by using other types of calculation methods. Thesignal obtained from the output of the summing means 240 forms a baseaddress. The base address is thus formed by squaring the input signalsof the calculation means 210, 220. In practice, the squaring of theinput signals corresponds to the calculation of the power of the inputsignals. Calculating an address according to the arrangement of theinvention is thus based on the level of the input signal.

The arrangement further comprises address correction means 250 inconnection with the summing means 240. The base address formed by thesumming means 240 is conveyed to the address correction means 250 thatcorrects the above calculated base address by means of an addresscorrection value. The base address corrected in the address correctionmeans 250 is used as an actual address (ADDRESS) of the corrector table.A corrector co-efficient is retrieved from the corrector table on thebasis of the corrector table address that has been formed, the correctorcoefficient compensating for signal distortions.

In practice, the address correction values in the address correctionmeans 250 are placed for example in a table 130. The address correctionvalues arranged in the input signal enable an optimal address to beformed for the distortion corrector table. The values of the addresscorrection table are preferably formed on the basis of the amplitudedistribution of the input signal. The effect of the modulation methodused is taken into account when generating values for the addresscorrection table. The optimal address enables the use of an optimalcorrector coefficient, whereby signal distortions can be accuratelycompensated for.

Although the invention has been described above with reference to theexamples illustrated in the attached drawings, it is obvious that theinvention is not restricted thereto but it can be modified in many wayswithin the scope of the inventive idea presented in the attached claims.

What is claimed is:
 1. A method for forming an address that is employedin signal predistortion to compensate for signal distortions by means ofcorrector coefficients placed in a corrector table, the correctorcoefficients being retrieved from the corrector table on the basis ofthe address, the method comprising: calculating a base address on thebasis of a level of an input signal; correcting the calculated baseaddress utilizing an address correction value according to an amplitudedistribution of the input signal; and calculating a corrector tableaddress on the basis of the calculated base address and the correctedbase address in an address correction table, wherein the base address iscalculated based on a sum of squaring at least one signal component ofthe input signal.
 2. The method as claimed in claim 1, wherein the baseaddress is calculated by calculating a result corresponding to thesquaring of the level of the input signal.
 3. The method as claimed inclaim 1, wherein the base address is calculated from an input signalcomposed of signal components in such a way that results correspondingto the squaring of single signal components are summed up.
 4. The methodas claimed in claim 1, wherein the input signal is composed of multiplesignal components, the signal components are squared separately, afterwhich the squared signal components are summed together.
 5. The methodas claimed in claim 1, wherein the base address is calculated based uponthe power of the input signal.
 6. The method as claimed in claim 1,wherein, when the method is used in association with differentmodulation methods, an address correction value is formed according to amodulation method used.
 7. The method as claimed in claim 1, wherein,when the method is used in association with different modulationmethods, the base address is corrected according to a manner required bythe modulation method used.
 8. The method as claimed in claim 1, whereinthe method for forming an address is employed in association withdigital predistortion.
 9. The method as claimed in claim 1, wherein thesignal components are I and Q signal components of a digital modulation.10. An arrangement for forming a corrector table address for use withsignal predistortion, the arrangement being used to compensate forsignal distortions using corrector coefficients placed in the correctortable, the corrector coefficients being retrieved from the correctortable on the basis of a base address, the arrangement comprising:calculation means that, on the basis of a received signal, calculate aresult that corresponds to squaring of the received signal; and anaddress correction table configured to correct the base addressaccording to the amplitude distribution of the received signal utilizingan address correction value, wherein the base address is calculatedbased on a sum of squaring at least one signal component of the inputsignal.
 11. An arrangement as claimed in claim 10, wherein thecalculation means calculate the base address by forming a result fromthe received signal, the result corresponding to the squaring of saidreceived signal.
 12. An arrangement as claimed in claim 10, wherein thereceived signal is composed of signal components, and the calculationmeans squares the signal components separately and the arrangementfurther comprises a summing means that sums the squared signalcomponents to produce a base address.
 13. An arrangement as claimed inclaim 10, wherein the calculation means calculates the base address onthe basis of the power level of the received signal.
 14. An arrangementas claimed in claim 10, wherein the address correction table is used tocorrect an address on the basis of which the corrector coefficient isretrieved from the corrector table, the corrector coefficient being usedto compensate for signal distortions.
 15. An arrangement as claimed inclaim 10, wherein the address correction table generates the addresscorrection values according to a predetermined modulation method.
 16. Anarrangement as claimed in claim 10, wherein the arrangement is used inassociation with digital predistortion.
 17. An arrangement as claimed inclaim 10, wherein the calculation means calculate the base address fromI and Q signal components of a digital modulation.
 18. A method forforming an address in signal predistortion to compensate for signaldistortions using corrector coefficients placed in a corrector table,the corrector coefficients being retrieved from the corrector table onthe basis of the formed address, the method comprising: calculating abase address on the basis of a received signal; conveying the baseaddress to an address correction table; and correcting the calculatedbase address according to an amplitude distribution of the receivedsignal using an address correction value in the address correction tableto produce an actual address for the corrector table, wherein the baseaddress is calculated based on a sum of squaring at least one signalcomponent of the input signal.
 19. A method for forming an addressduring signal predistortion used to compensate for signal distortionsusing corrector coefficients placed in a corrector table, the correctorcoefficients being retrieved from the corrector table on the basis ofthe address, the method comprising: calculating a base address on thebasis of a received signal; conveying the base address according to anaddress correction table; correcting the base address according to anamplitude distribution of the received signal in the address correctiontable; and forming an address used as an actual address for thecorrector table based on the base address in the address correctiontable, wherein the base address is calculated based on a sum of squaringat least one signal component of the input signal.
 20. An arrangementfor forming a corrector table address during signal predistortion tocompensate for signal distortions using corrector coefficients placed ina corrector table, the corrector coefficients being retrieved from thecorrector table on the basis of the correction table address, thearrangement comprising: means for forming a base address on the basis ofa received signal; and an address correction table for forming anaddress according to an amplitude distribution of the received signal bycorrecting the base address by means of an address correction value, theaddress being used as an actual address for the corrector tables,wherein the base address is calculated based on a sum of squaring atleast one signal component of the input signal.
 21. An arrangement forforming a corrector table address used for signal predistortion tocompensate for signal distortions using corrector coefficients placed ina corrector table, the corrector coefficients being retrieved from thecorrector table on the basis of the corrector table address, thearrangement comprising: means for forming a base address on the basis ofa received signal; and an address correction table for forming anaddress according to an amplitude distribution of the received signalbased on the base address, the address being used as an actual addressfor the corrector table, wherein the base address is calculated based ona sum of squaring at least one signal component of the input signal. 22.A method of forming an address that is employed in signal predistortionto compensate for signal distortions by means of corrector coefficientsplaced in a corrector table, the corrector coefficients being retrievedfrom the corrector table on the basis of the address, the methodcomprising: calculating a base address as a sum of squared values of Iand Q part of an input signal; correcting the calculated base addressutilizing an address correction value according to an amplitudedistribution of the input signal; and calculating a corrector tableaddress on the basis of the calculated base address and the correctedbase address in an address correction table, wherein the base address iscalculated based on a sum of squaring at least one signal component ofthe input signal.
 23. An arrangement for forming an address that isemployed in signal predistortion to compensate for signal distortions bymeans of corrector coefficients placed in a corrector table, thecorrector coefficients being retrieved from the corrector table on thebasis of the address, the arrangement comprising: a squaring elementconfigured to square an I signal of an input signal; a squaring elementconfigured to square a Q signal of an input signal; a summer configuredto form a base address as a sum of squared values of I and Q part of aninput signal; and address correction table configured to calculate acorrector table address on the basis of the calculated base address,wherein the base address is calculated based on a sum of squaring atleast one signal component of the input signal.